Reducing collision probability for VoIP packets

ABSTRACT

A mobile communications device is described that includes a processor configured to establish a network connection with a server in a network and a data generator under the control of the processor that is configured to periodically generate a data unit at a rate having a fixed period. The mobile communications device also includes a transceiver configfured to transmit, via the network connection, the data unit during one of the plurality of slots, each slot being an adjacent fractional portion of the fixed period.

BACKGROUND

1. Field

The present disclosure relates generally to telecommunications, and moreparticularly, to systems and methods for controlling and managing accessto a wireless network.

2. Background

The demand for wireless information services has led to the developmentof an ever increasing number of wireless networks. CDMA2000 1x is justone example of a wireless network that provides wide area telephony anddata services. CDMA2000 1x is a wireless standard promulgated by theThird Generation Partnership Project 2 (3GPP2) using code divisionmultiple access (CDMA) technology. CDMA is a technology that allowsmultiple users to share a common communications medium usingspread-spectrum processing. A competing wireless network that iscommonly employed in Europe is Global System for Mobile Communications(GSM). Unlike CDMA2000 1x, GSM uses narrowband time division multipleaccess (TDMA) to support wireless telephony and data services. Someother wireless networks include General Packet Radio Service (GPRS)which supports high speed data services with data rates suitable fore-mail and web browsing applications, and Universal MobileTelecommunications System (UMTS) which can deliver broadband voice anddata for audio and video applications.

These wireless networks can generally be thought of as wide areanetworks employing cellular technology. Cellular technology is based ona topology in which the geographic coverage region is broken up intocells. Within each of these cells is a fixed base transceiver station(BTS) that communicates with mobile users. A base station controller(BSC) is typically employed in the geographic coverage region to controlthe BTSs and route communications to the appropriate gateways for thevarious packet-switched and circuit-switched networks.

As the demand for wireless information services continue to increase,mobile devices are evolving to support integrated voice, data, andstreaming media while providing seamless network coverage between widearea cellular networks and wireless local area networks (LAN). WirelessLANs generally provide telephony and data services over relatively smallgeographic regions using a standard protocol, such as IEEE 802.11,Bluetooth, or the like. The existence of wireless LANs provides a uniqueopportunity to increase user capacity in a wide area cellular network byextending cellular communications to the unlicensed spectrum using theinfrastructure of the wireless LAN.

However, unlike cellular services, many wireless LAN technologies allowall potential transmitting devices random access to the broadcast mediumwithout guaranteeing particular Quality of Service (QoS) levels. Thus,for certain applications such as Voice over IP (VoIP), delays may beintroduced between communicating users that are, at the least, minorannoyances and, at the worst, unacceptable from the user's viewpoint.Because many such wireless LANs also employ random back-off algorithmsto avoid subsequent contention between the users, these delays (andtherefore performance) only worsen as more VoIP users associate with aparticular wireless LAN.

Some techniques for addressing this problem have been developed in thepast with respect to 802.11 wireless LANs. These techniques havetypically included assigning a priority to VoIP traffic and thenhandling that traffic outside of the implemented standard using tacticssuch as modifying the contention window, modifying the interframespacing, jamming or overriding other traffic; not requiring anacknowledgement (ACK), using a zero-back-off setting, and transmittingVoIP packets ahead of any data packets waiting in the transmissionqueue.

Regardless of the merits of any of these techniques, continuedimprovement in the QoS of VOIP applications within a wireless LANremains desirable and it is preferable that such improvement beaccomplished without deviating from already established communicationsstandards such as, for example, 802.11.

SUMMARY

One aspect of a mobile communications device is disclosed that relatesto a processor configured to establish a connection with a server in arandom access network and a data generator under the control of theprocessor that is configured to periodically generate a data unit at arate having a fixed period. The mobile communications device alsoincludes a transceiver configured to transmit to the server the dataunit during a particular transmission slot in each of the fixed periods.

Another aspect of a mobile communications device is described thatrelates to a processor configured to establish a network connection overa random access network and a vocoder controlled by the processor thatis configured to generate a voice packet at a rate having a fixedperiod. Furthermore, the processor configured to buffer the voice packetuntil a transmit time. This mobile communications device furtherincludes a transceiver, in communications with the buffer, that isconfigured to transmit the voice packet at the transmit timecorresponding to a particular time slot in each of the fixed periods.

Yet another aspect of a mobile communications device is described thatrelates to a processor configured to identify a particular transmissionslot in a transmission period. The processor is further configured toestablish a network connection with a server over a random accessnetwork, and periodically generate a data unit at a period substantiallyequal to the transmission period. In accordance with this aspect of thedevice a transceiver is configured to transmit, to the server, the dataunit during the particular transmission slot.

It is understood that other embodiments of the present invention willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein it is shown and described only variousembodiments of the invention by way of illustration. As will berealized, the invention is capable of other and different embodimentsand its several details are capable of modification in various otherrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawings and detailed description are to beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of a wireless communications system are illustrated byway of example, and not by way of limitation, in the accompanyingdrawings, wherein:

FIG. 1 is a conceptual block diagram of an embodiment of a wirelesscommunications system;

FIG. 2 is a functional block diagram illustrating an example of a mobiledevice capable of supporting both cellular and wireless LANcommunications;

FIG. 3 is a conceptual block diagram of a wireless communications systemhaving multiple mobile devices;

FIG. 4 is a flow diagram illustrating the functionality of assigning aslot index to a mobile device in accordance with the principles of thepresent invention; and

FIG. 5 is a flow diagram illustrating the functionality of transmittinga packet from a mobile device at a time based on an assigned slot index.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various embodiments of theinvention and is not intended to represent the only embodiments in whichthe invention may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof the invention. However, it will be apparent to those skilled in theart that the invention may be practiced without these specific details.In some instances, well known structures and components are shown inblock diagram form in order to avoid obscuring the concepts of theinvention.

In the following detailed description, techniques will be described forselecting a network in a multiple network environment. A number oftechniques will be described in the context of a mobile communicationsdevice traveling through a wide area cellular network with one or morewireless LANs dispersed throughout the cellular coverage region. Themobile communications device may be any suitable device capable ofwireless telephony or data communications, such as a cellular phonedesigned for operation in a CDMA2000 1x network. The mobilecommunications device may be capable of employing any suitable protocolfor accessing a wireless LAN, including, by way of example, IEEE 802.11.While these techniques may be described in the context of a cellularphone capable of communicating with an IEEE 802.11 network, thoseskilled in the art will readily appreciate that these techniques can beextended to other mobile communication devices capable of accessingmultiple networks. For instance, these techniques may be applied to amobile communications device capable of switching between a CDMA2000 1xnetwork and a GSM network. Alternatively, these techniques may beapplied to a mobile communications device capable of accessing a singlenetwork, such as a IEEE 802.11 phone. The IEEE 802.11 phone may beconfigured to connect to the wireless LAN only if certain parametersindicate that the service quality is acceptable. Accordingly, anyreference to a cellular phone capable of communicating with an IEEE802.11 network, or any other specific embodiment, is intended only toillustrate various aspects of the present invention, with theunderstanding that these aspects have a wide range of applications.

FIG. 1 is a conceptual block diagram of an embodiment of a wirelesscommunications system. A mobile device 102 is shown moving through awide area cellular network 104 by a series of broken lines. The cellularnetwork 104 includes a BSC 106 supporting a number of BTSs dispersedthroughout the cellular coverage region. A single BTS 108 is shown inFIG. 1 for simplicity of explanation. A mobile switching center (MSC)110 may be used to provide a gateway to a public switched telephonenetwork (PSTN) 112. Although not shown in FIG. 1, the cellular network104 may employ numerous BSCs each supporting any number of BTSs toextend the geographic reach of the cellular network 104. When multipleBSCs are employed throughout the cellular network 104, the MSC 110 mayalso be used to coordinate communications between the BSCs.

The mobile device 102 is initially shown in location A in FIG. 1. As themobile device 102 moves through the cellular network 104 from location Ato location B, it comes within the coverage region of a wireless LAN114. The wireless LAN 114 may be an IEEE 802.11 network, or any othersuitable network. The wireless LAN 114 includes an access point 116 forthe mobile device 102 to communicate with an IP network 118. A server120 may be used to interface the IP network 118 to the MSC 110, whichprovides a gateway to the PSTN 112.

When power is initially applied to the mobile device 102, it registerswith either the cellular network 104 or the wireless LAN 114.“Registration” refers to a process whereby the mobile device 102 tellsthe MSC 110 to route calls from the PSTN 112 through a particularnetwork. The decision to register with a particular network may varydepending on the specific application and overall design constraints. Byway of example, the mobile device 102 may be configured to register withthe wireless LAN 114 if the service quality is acceptable. By routingall calls to the mobile device 102 through the wireless LAN 114,valuable cellular bandwidth may be freed up for other mobile users.

As the mobile device 102 moves through the cellular network 104 fromlocation A to location B in the depicted embodiment, it begins to detecta beacon from the access point 116. Once the mobile device 102 detectsthe beacon, a radio connection may be established between the two bymeans well known in the art. The mobile device 102 then obtains the IPaddress of the server 120. The mobile device 102 may use the services ofa Domain Name Server (DNS) to determine the server's IP address. Thedomain name of the server 120 may be delivered to the mobile device 120over the cellular network 104. With the IP address, the mobile device102 can establish a network connection with the server 120. Once thenetwork connection is established, information from the server 120 canbe used in conjunction with local measurements to determine whether toupdate its registration with the MSC 110 so that future calls are routedthrough the wireless LAN 114.

FIG. 2 is a functional block diagram illustrating an example of a mobiledevice capable of supporting both cellular and wireless LANcommunications. The mobile device 102 may include a cellular transceiver202 and a wireless LAN transceiver 204. In at least one embodiment ofthe mobile device 102, the cellular transceiver 202 is capable ofsupporting CDMA2000 1x communications with a BTS (not shown), and thewireless LAN transceiver 204 is capable of supporting IEEE 802.11communications with an access point (not shown). Those skilled in theart will readily appreciate, however, that the concepts described inconnection with the mobile device 102 can be extended to other cellularor wireless LAN technologies. Each transceiver 202, 204 is shown with aseparate antenna 206, 207, respectively, but the transceivers 202, 204could share a single broadband antenna. Each antenna 206, 207 mayinclude one or more radiating elements.

The mobile device 102 is also shown with a processor 208 coupled to boththe cellular transceiver 202 and the wireless LAN transceiver 204,however, separate processors may be used for each transceiver inalternative embodiments of the mobile device 102. The processor 208 maybe implemented as hardware, firmware, software, or any combinationthereof. By way of example, the processor 208 may include amicroprocessor (not shown). The microprocessor may be used to supportsoftware applications that, among other things, (1) control and manageaccess to the cellular network and wireless LAN, and (2) interface theprocessor 208 to the keypad 210, display, 212, and other user interfaces(not shown). The processor 208 may also include a digital signalprocessor (DSP) (not shown) with an embedded software layer thatsupports various signal processing functions, such as convolutionalencoding, modulation and spread-spectrum processing. The DSP may alsoimplement a vocoder 211 to provide functions to support telephonyapplications. The manner in which the processor 208 is implemented willdepend on the particular application and the design constraints imposedon the overall system. Those skilled in the art will recognize theinterchangeability of hardware, firmware, and software configurationsunder these circumstances, and how best to implement the describedfunctionality for each particular application.

The processor 208 may be configured to execute a media access algorithmthat is used to determine when the mobile device 102 should transmitVoIP packets to the access point 116. This algorithm may be implementedas one or more software applications supported by the microprocessorbased architecture discussed earlier. Alternatively, this algorithm maybe a separate module from the processor 208 implemented in hardware,software, firmware, or any combination thereof. Depending on thespecific design constraints, the media access algorithm could beintegrated in any entity in the mobile device 102, or distributed acrossmultiple entities in the mobile device 102.

FIG. 3 depicts a wireless LAN 300 with an access point 302 that issimilar to that of access point 116 of FIG. 1. Furthermore, the accesspoint 302 includes a connection to the server 120 through the IP network118. Although not explicitly depicted in FIG. 3, the server 120 is incommunication with the MSC 110 as shown in FIG. 1. FIG. 3 is provided asan example of a wireless LAN 300 that has multiple devices associatedtherewith. For example, a portable computer 308 and two mobile devices304, 306 are shown as being associated with the access point 302. Inparticular, the mobile device 304, 306 may be substantially similar tothe mobile device 102 described earlier with reference to FIGS. 1 and 2.

One exemplary type of wireless LAN may be provided by an access point302 that conforms to an 802.11 standard. As is known to one of ordinaryskill, the devices 304, 306 and 308 communicate by exchanging packetsover the air with the access point 302 according to the standardizedprotocol. The transmission of packets is not coordinated among thedifferent devices 304, 306, 308 or controlled by the access point 302.Instead, any device is free to broadcast at any time they have dataavailable. Furthermore, the 802.11 standard, as well as many otherstandards allowing random access to the broadcast medium (i.e., a randomaccess network), provides a mechanism to detect collisions that permitsdevices to re-transmit packets that may have collided. As used herein, a“collision” may include the occurrence of any phenomenon on thebroadcast media that prevents a packet from being accurately received atan intended destination. Typically, a collision occurs when a deviceattempts to broadcast a packet during the same time that another deviceis broadcasting a packet.

With only a few devices 304, 306, 308 using the access point 302, thelikelihood of a collision occurring is small enough that users usuallycannot detect any impact on performance. However, as more devices areassociated with the wireless LAN 300, the likelihood of collisionsincrease and performance effects can become noticeable. Additionally,some applications are more sensitive to any delays that are caused bycollisions. For example, users who are utilizing the wireless LAN 300for VoIP applications will notice delays in the voice transmissions muchmore easily than users who are utilizing typical data applications.

In addition to some applications, such as VoIP, being more sensitive todelays, the algorithms for handling collisions can, themselves,contribute to the performance degradation caused by collisions for allapplications. Many wireless LAN standards employ random back-offalgorithms which prevent a device from re-transmitting a packet until arandom time period has expired. If a collision occurs again, then themaximum value for this random time period is lengthened. Thus, as moredevices cause more collisions, the random back-off periods can grow inlength and thereby, cause additional delays in transmitting packets in acrowded wireless LAN. Furthermore, the only indication that a collisionoccurred is typically the absence of receiving an ACK packet. Thus,devices must wait for the time-out period of receiving the ACK packetbefore knowing that a collision occurred and that a re-transmission isneeded.

FIGS. 4 and 5 depict an algorithmic approach to decreasing the collisionprobability for transmitted packets that may, for example, be VoIPpackets. This particular algorithmic approach is described herein withreference to a CDMA and wireless LAN device similar to that of FIG. 2.However, it may be appreciated that utilization of othertelecommunications technologies is also contemplated within the scope ofthe present invention.

Typically, within a VoIP application, a vocoder 211 generates a voicepacket at fixed intervals. For example, in the example described below,the vocoder generates a voice packet every 20 msecs. Once this packet isgenerated, it is passed through the MAC layer onto the PHY layer ascommonly known in the art. If two or more VoIP devices are producingvoice packets at substantially the same rate (e.g., every 20 msecs) andthey happen to be doing so within a closely spaced interval, then thereis a likelihood that their respective voice packets will collide.Furthermore, because they will continue to generate voice packets at thesame rate, their subsequent voice packets will likely collide as well.The flowchart of FIG. 4 illustrates an exemplary algorithm for reducingthe likelihood of collision between voice packets by coordinated thetransmission of the packets from the different devices. In the presentdescription a vocoder and voice packets are described merely by way ofexample and one of ordinary skill will recognize that other datagenerators that generate data units or packets at a fixed period arealso contemplated.

In step 402, a mobile device (e.g., 304) connects, or registers, withthe server 120 as explained earlier with respect to FIG. 1. Part of thisregistration process involves identifying the wireless LAN 300. From theidentification of the wireless LAN 300, the server determines, in step404, other VoIP devices (e.g., 306) on that same wireless LAN 300.

In addition to the server 120 maintaining a list of the devices on thewireless LAN 300, the server also maintains a respective slot indexvalue that is associated with each of the devices. Based on this slotindex information, the server 120, in step 406, identifies a free slotand assigns it to the device being registered. A “slot” refers to aportion of a time period in which a voice packet may be transmitted. Forexample, in the earlier described vocoder, there is a 20 msec intervalbetween transmission of voice packets. This time interval may be used totransmit the voice packet before another voice packet is generated.Because successful transmission of a voice packet may only require 1msec or less, multiple voice packets may be transmitted from differentdevices in a non-overlapping manner during that 20 msecs. If the 20 msecinterval is broken into 16 slots for example, then 16 different 1.25 mswindows are available for transmitting a voice packet during eachinterval. A “slot index” simply identifies one of the possible 16 slots.One of ordinary skill will recognize that the 20 msec value, the 1 msecvalue, and the value of 16 slots are merely provided by way of exampleand that these particular values may be modified in a variety ofdifferent ways to define time intervals, different numbers of slots anddifferent slot lengths during which a voice packet may be transmitted bydifferent devices. Furthermore, the packet being transmitted does notnecessarily have to be a voice packet. Transmission of regularlyoccurring data packets in other applications, especially those thatwould suffer from packet delay, will also benefit from the techniquesdescribed herein.

In step 408, the server 120 returns a message to the device thatidentifies the device's assigned slot index. The information returned bythe server 120 may identify the slot index simply as a value (e.g., “5”)or in a manner such as “5 of 20”. This latter method informs the devicenot only of its slot index but also the maximum number of slots. In thisway, the number of slots, and therefore the slot duration, may bevariable depending on the information returned to the device. FIG. 5depicts a flowchart of an algorithm by which devices may use the slotindex information to reduce the probability of collision between voicepackets.

In step 502, the vocoder generates a voice packet every 20 msec. Asdescribed earlier, this time interval is typically a function of thevocoder 211 and may vary. Instead of immediately passing the voicepacket to the wireless LAN MAC layer for transmission, the devicestores, in step 504, the voice packet in a buffer 213. The device thenmeasures time from when the voice packet is generated. When the time forthe slot arrives that has been assigned to the device, the bufferedvoice packet is passed, in step 506, to the 802.11 MAC layer fortransmission.

Coordinating the transmission of voice packets in this mannereffectively reduces the probability of collision between packets sent bydifferent devices. The timekeeping that takes place within the device todetermine when their assigned slot is occurring may be accomplished by anumber of different methods. For example, the different devices (e.g.304, 306) may utilize available CDMA signals to synchronize to a commonclock so that the 20 msec intervals occur substantially at the same timefor each device. If the 20 msec (or some other length) intervals occurat different times for different devices, then the CDMA signals maystill be used to extract enough information for a device to determinethe start of their interval and, thereby, determine when each slot inthat interval starts. Alternatively, an IP-based protocol such as thenetwork time protocol (NTP) may be available through the wireless LAN toprovide time synchronization information for the different devices.Other alternative methods are also contemplated that allow a device todetermine, based on an assigned slot index, when in each regularlyoccurring interval to transmit a voice, or other, packet.

A number of variations to the specific behaviors and steps described inthe above examples may be made without departing from the scope of thepresent invention. For example, as described, the server determines anunassigned slot index to assign to a new device being registered. Theremay be some instances, however, in which there are more devices thanslots; in such cases, the slots indices may be distributed so that asfew devices as possible share any particular slot index. Additionally,the access point 302, instead of the server 120, may assign the slotindex to each device. The assignment of a slot index may alternativelyoccur in a random fashion or be based on some data point that variesamong devices such as, for example, the time of day when a deviceregisters or the last byte of the device's MAC address. In addition toVoIP traffic, other regularly occurring packet transmissions may besynchronized in a manner similar to that described herein in order toreduce the probability of packet collisions. The term “packet” has anaccepted meaning within networks known as packet network; however, asused herein the term “packet” may refer to any discrete data unit thatis assembled for transmission via a network.

The various illustrative logical blocks, modules, circuits, elements,and/or components described in connection with the embodiments disclosedherein may be implemented or performed with a general purpose processor,a digital signal processor (DSP), an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA) or otherprogrammable logic component, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computingcomponents, e.g., a combination of a DSP and a microprocessor, aplurality of microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

The methods or algorithms described in connection with the embodimentsdisclosed herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. A storagemedium may be coupled to the processor such that the processor can readinformation from, and write information to, the storage medium. In thealternative, the storage medium may be integral to the processor.

The previous description is provided to enable any person skilled in theart to practice the various embodiments described herein. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments. Thus, the claims are not intended to belimited to the embodiments shown herein, but is to be accorded the fullscope consistent with the language claims, wherein reference to anelement in the singular is not intended to mean “one and only one”unless specifically so stated, but rather “one or more.” All structuraland functional equivalents to the elements of the various embodimentsdescribed throughout this disclosure that are known or later come to beknown to those of ordinary skill in the art are expressly incorporatedherein by reference and are intended to be encompassed by the claims.Moreover, nothing disclosed herein is intended to be dedicated to thepublic regardless of whether such disclosure is explicitly recited inthe claims. No claim element is to be construed under the provisions of35 U.S.C. §112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

1. A mobile communications device, comprising: a processor configured toestablish a connection with a server in a random access network; a datagenerator under the control of the processor and configured toperiodically generate a data unit at a rate having a fixed period; and atransceiver under control of the processor and configured to transmitthe data unit to the server during a particular time slot in each of thefixed periods.
 2. The mobile communications device of claim 1, whereinthe particular slot is one of a plurality of adjacent slots, each of theplurality of slots has associated therewith a respective slot index. 3.The mobile communications device of claim 2, wherein the plurality ofslots are equal in size.
 4. The mobile communications device of claim 2,wherein the plurality of slots, in aggregate are substantially equal tothe fixed period.
 5. The mobile communications device of claim 2,wherein each of the plurality of slots is of sufficient length to permitthe data unit to be transmitted before a beginning of an adjacent slot.6. The mobile communications device of claim 2, wherein the processor isfurther configured to receive from the server a particular slot index.7. The mobile communications device of claim 6, wherein the particularslot corresponds to the slot associated with the particular slot index.8. The mobile communications device of claim 1, wherein the data unitcomprises a voice over IP packet.
 9. The mobile communications device ofclaim 1, wherein the network comprises a wireless LAN.
 10. A mobilecommunications device comprising: a processor configured to establish aconnection with a server in a random access network; a vocodercontrolled by the processor and configured to generate a voice packet ata rate having a fixed period; the processor further configured to bufferthe voice packet until a transmit time; and a transceiver, incommunications with the buffer and controlled by the processor, saidtransceiver configured to transmit the voice packet at the transmittime, wherein the transmit time corresponds to a particular time slot ineach of the fixed periods.
 11. The mobile communications device of claim10, wherein the particular slot is one of a plurality of slots, each ofthe plurality of slots has associated therewith a respective slot index.12. The mobile communications device of claim 11, wherein each of theplurality of slots is of sufficient length to permit the voice packet tobe transmitted before a beginning of an adjacent slot.
 13. The mobilecommunications device of claim 11, wherein the processor is furtherconfigured to receive a particular slot index from a server via thenetwork connection.
 14. The mobile communications device of claim 13,wherein the particular time slot corresponds to the slot associated withthe particular slot index.
 15. The mobile communications device of claim10, wherein the network comprises at least a wireless LAN.
 16. A mobilecommunications device comprising: a processor configured to identify aparticular transmission slot; a processor configured to establish aconnection with a server in a random access network, the processor beingfurther configured to periodically generate a data unit at asubstantially fixed period and identify a particular transmission slotin each of the fixed periods; and a transceiver configured to transmit,to the server, the data unit during the particular transmission slot.17. The mobile communications device of claim 16, wherein the processoris further configured to identify each of a plurality of transmissionslots using an associated slot index value.
 18. The mobilecommunications device of claim 17, wherein the processor is furtherconfigured to receive an assigned slot index from the server.
 19. Themobile communications device of claim 18, wherein the particulartransmission slot corresponds to the slot associated with the assignedslot index.
 20. The mobile communications device of claim 16, whereinthe network comprises at least a wireless LAN connection.
 21. A mobilecommunications device, comprising: means for establishing a networkconnection with a server over a random access network and periodicallygenerating a data unit at a rate having a fixed period; and means fortransmitting the data unit during a particular transmission slot.